Unraveling the Effect\nof Oxygen Vacancy on WO<sub>3</sub> Surface for Efficient NO<sub>2</sub> Detection at Low Temperature

Oxygen vacancies (V<sub>O</sub>) in metal oxide semiconductors\nplay an important role in improving gas-sensing performance of chemiresistive\ngas sensors. Nonetheless, there is still a lack of clear understanding\nof the inherent mechanism of the influence of oxygen vacancies on\ngas sensing due to generally focusing on the concentration of V<sub>O</sub>. Herein, oxygen vacancies were rationally modulated in WO<sub>3</sub> nanoflower structures via an annealing process, resulting\nin a transformation of V<sub>O</sub> from neutral (V<sub>O</sub><sup>0</sup>) to a doubly ionized (V<sub>O</sub><sup>2+</sup>) state. Density\nfunctional theory (DFT) calculations indicate that V<sub>O</sub><sup>2+</sup> is significantly more efficient\nthan V<sub>O</sub><sup>0</sup> for\nNO<sub>2</sub> detection in competition with atmospheric O<sub>2</sub>. Benefiting from a high concentration of V<sub>O</sub><sup>2+</sup>, the WO<sub>3</sub>-450 (WO<sub>3</sub> annealed at 450 °C) sensor exhibits excellent sensing performance\nwith an ultrahigh sensitivity (3674.1 to 5 ppm NO<sub>2</sub>), superior\nselectivity, and long-term stability (one month). Furthermore, the\nsensor with the wide range of concentration detection not only can\ndetect NO<sub>2</sub> gas with parts per million (ppm) but also can\ndetect NO<sub>2</sub> with parts per billion (ppb) level concentration,\nwith a high sensibility reaching 2.8 to 25 ppb NO<sub>2</sub> and\nover 100 to 100 ppb NO<sub>2</sub>. This study elucidates the oxygen\nvacancy mediated sensing mechanism toward NO<sub>2</sub> and provides\nan effective strategy for the rational design of gas sensors with\nhigh sensing performance.